The present invention relates to a method for producing fast-drying multi-component waterborne coating compositions, particularly traffic paints or road markings. The invention also relates to the compositions of fast-drying multi-component waterborne coatings, particularly traffic paints or road markings. Used herein, the term xe2x80x9cmulti-componentxe2x80x9d refers to traffic paints having two or more components applied to a substrate in one or more steps.
One of the many important features of coatings in general, and road markings or traffic paints in particular, is the speed at which they dry on the surface of a particular substrate after application. For instance, the drying speed of a traffic paint dictates the length of the period of disruption to road traffic during application of the paint to road surfaces, and subsequent drying. The trend is to demand shorter and shorter disruptions of traffic flow, and to meet this demand by using faster drying paint. Used herein, the terms xe2x80x9ccoatingxe2x80x9d and xe2x80x9cpaintxe2x80x9d will be used interchangeably and referred to as a general class including traffic paints and road markings. In addition, the terms xe2x80x9ctraffic paintxe2x80x9d and xe2x80x9croad markingxe2x80x9d are used interchangeably herein.
Solvent-based fast-drying coatings are based on organic polymeric resins (also frequently called binders) dissolved, suspended or otherwise dispersed in relatively low-boiling organic solvents. Low-boiling volatile organic solvents evaporate rapidly after application of the paint on the road to provide the desired fast drying characteristics of a freshly applied road marking. However, in addition to releasing volatile organic solvents into the environment, this type of paint formulation tends to expose the workers to the vapors of the organic solvents. Because of these shortcomings and increasingly stringent environmental mandates from governments and communities, it is highly desirable to develop more environmentally friendly coatings or paints while retaining fast drying properties and/or characteristics.
A more environmentally friendly coating uses water based, i.e., waterborne, rather than solvent based polymers or resins. Coating formulations, both solvent based and waterborne, include binders. The terms xe2x80x9cbinderxe2x80x9d and xe2x80x9cbinder polymerxe2x80x9d used herein refer to polymers that are included in the coating composition and that augment or participate in film formation and in the composition of the resultant film. Binder polymers typically have Tg values in the range xe2x88x9210xc2x0 C. to 70xc2x0 C., because those having Tg values below xe2x88x9210xc2x0 C. tend to have poor resistance to dirt pick-up and those having Tg values above 70xc2x0 C. usually display diminished ability to form films. In certain applications, however, the lower limit for Tg can be even lower than xe2x88x9210xc2x0 C. For example, the binder polymers used in roof coatings can have glass transition temperatures as low as xe2x88x9240xc2x0 C. Used herein, Tg is an abbreviation for glass transition temperature. Primarily due to a combination of high boiling point, high latent heat of vaporization, high polarity, and strong hydrogen bonding of water, drying times of waterborne paints or coatings are generally longer than those exhibited by the organic solvent based coatings. The drying time strongly depends on the relative humidity of the atmosphere in which the coatings are applied. A waterborne paint may take several hours or more to dry in high humidity. The problem of retarded drying rate is especially aggravated for thick film (greater than about 500xcexc) traffic markings. Long drying times severely limit the desirability of using waterborne paints, particularly traffic paints and road marking paints because of longer traffic disruptions.
In an attempt to produce waterborne coating compositions with shorter drying times, i.e., xe2x80x9cfast-dryingxe2x80x9d coatings, methods utilizing salt, or acid, or combinations thereof to induce coagulation have been devised, as have pH sensitive binder systems.
EP-A-0066108 discloses an aqueous road marking composition in which the binder is a mixture of a pure acrylic resin, a carboxylated styrene/dibutyl fumarate copolymer and a polymeric, polyfunctional amine such as polypropylene imine. This application states that the disclosed compositions are not storage stable beyond 48 hours after which more polyfunctional amine must be added to restore activity. This low storage stability is unacceptable for most uses.
EP-B-0322188 discloses aqueous coating compositions comprising film forming latex polymer, weak base-functional synthetic latex polymer and volatile base. However, the weak base-functional polymer therein is water-insoluble as a consequence of crosslinking or high molecular weight. Such water-insolubility may cause the weak base moieties to be less readily available to interact with the latex polymer particles and the species (e.g., anionic surfactants) that stabilize them. This reduced availability may occur because a significant portion of the weak base moieties are buried beneath the surface of the water-insoluble particles or because insoluble particles are inherently limited in there ability to disperse their functionality uniformly within the coating composition.
EP-B-0409459 discloses an aqueous coating composition including an anionically stabilized emulsion polymer having Tg no lower than 0xc2x0 C., a polyamine functional polymer, and a volatile base in an amount such that the composition has a pH where substantially all of the polyamine functional polymer is in a non-ionic state, and wherein more than 50% by weight of the polyamine functional polymer will be soluble at pH values of 5 to 7 on evaporation of the volatile base. In the non-ionic state (i.e., deprotonated), polyamine interaction with the anionically stabilized emulsion and any other anionic ingredients which may be present in the composition is eliminated. The volatile base must be volatile enough to be released under air drying conditions. In the absence of the volatile base, the protonated amine moieties interact with the anionic ingredients to destabilize the coating composition.
U.S. Pat. No. 5,804,627 discloses methods of producing fast drying coatings on exterior surfaces that include applying on those surfaces an aqueous composition including an anionically stabilized emulsion polymer having a Tg greater than about 0xc2x0 C., a polyamine functional polymer having from about 20% to about 100% of the monomer units by weight containing an amine group, and an amount of volatile base sufficient to raise the pH of the composition to a point where essentially all of the polyamine functional polymer is in a non-ionic state, and evaporating the volatile base to produce the coating.
U.S. Pat. No. 5,922,398 discloses waterborne coating compositions containing latex particles having pendant amine-functional groups. The latex particles have Tg greater than about 0xc2x0 C. and are capable of film formation at application temperatures. A amount of base (e.g., ammonia) is added to raise the pH of the composition to a point where essentially all of the amine functional groups are in a non-ionic state. Also disclosed are methods of producing fast drying coatings on suitable substrates by application of the coating compositions. Upon formation of a film, the base evaporates, allowing the pendant amine moieties to become protonated. The resultant pendant ammonium moieties then interact with anionic surfactants to destabilize the aqueous system and, thereby, speed drying.
U.S. Pat. No. 5,824,734 discloses a waterborne coating composition including an amine functional latex polymer having 0.1 to 5% by weight, based on solid weight of polymer, of a secondary or tertiary amino acrylate, a crosslinking monomer, and less than 5% by weight of hydrophilic monomers. The composition also includes mineral pigments. The amine functional latex polymer is prepared at pH of at least 7. Following polymerization, the pH is adjusted upward, preferably to between 8 and 9.5, to maintain the stability of the system. Upon reducing the pH, the stability of the dispersion of polymer particles and mineral pigment particles is decreased, leading to precipitation of the polymer and mineral particles and drying.
While all of the above mentioned patents represent improvements in drying speed for waterborne coating systems, still further reduced drying times are needed, particularly for thick film (i.e., thickness greater than 500 microns, particularly 1 to 3 millimeters, mm) traffic paints and traffic markings.
U.S. Pat. No. 5,947,632 discloses waterborne coating compositions including a number of general categories of materials including talc, hollow sphere polymer, a solid polymer (e.g., ion exchange resin beads in acid, sodium or potassium form) and inorganic compounds (e.g., inorganic superabsorbent gel, Sumica gel). These materials share the characteristic that they speed the drying of coatings when applied either in the same first step with the waterborne binder, or in a subsequent step. U.S. Pat. No. 5,947,632 also discloses incorporation of glass beads into the waterborne coating compositions. Glass beads impart retro-reflective characteristics to traffic paints and traffic markings, and can also serve as fillers for coating compositions. Other additives such as anti-skid material are also disclosed. While the coating compositions and methods of application to substrates of U.S. Pat. No. 5,947,632 provide shorter drying times, the methods of applying the materials that augment drying (e.g., ion exchange resin beads) result in coatings having surfaces from which those materials protrude. Although such protrusions can be desirable for glass beads due to enhancement of retro-reflection, protrusion of, for example, colored, or opaque ion exchange resin beads can cause reduction in such desirable characteristics as whiteness. Further, a portion of these absorber particles may bounce off, or otherwise disengage from, the coating before contributing fully, or partially, to accelerating the drying of the coating.
Throughout this document, the term xe2x80x9cabsorberxe2x80x9d will be used to refer to the general class of materials that includes hollow sphere polymer, ion exchange resin beads (e.g., in acid form, in base form, in salt form, in partially neutralized form, or in mixed salt form), and absorbent inorganic compounds (e.g., inorganic superabsorbent gel, Sumica gel), including talc.
For a multi-component coating composition, we have unexpectedly discovered that when the absorber is applied in a step that precedes a step in which binder is applied, the surface of the dried coating (e.g., road marking) is smoother and drying times are shortened. In this regard, application of the multi-component coating composition to form xe2x80x9csandwichxe2x80x9d structures is particularly efficient, and preferred. Used herein, the terms xe2x80x9csandwichxe2x80x9d and xe2x80x9csandwich structurexe2x80x9d refer to films formed by applying the absorber in a step that is preceded by at least one step of applying a binder containing composition, and that is followed by at least one step of applying a binder containing composition. This definition of xe2x80x9csandwichxe2x80x9d and xe2x80x9csandwich structurexe2x80x9d further extends to films formed by combining the first two steps above into a single step such that the absorber and binder are applied simultaneously, or nearly simultaneously, followed by applying a binder containing composition. We have further discovered unexpectedly that molecular sieves, non-porous carbonaceous materials, porous carbonaceous materials, and superabsorbent polymers (abbreviated SAP or SAPs herein) also speed the drying of coating compositions. These molecular sieves, non-porous carbonaceous materials, porous carbonaceous materials, and superabsorbent polymers will also be referred to herein as xe2x80x9cabsorbersxe2x80x9d.